We show the design, fabrication and testing of micro/nanobiosensor devices based on optical waveguides in a highly sensitive interferometric configuration and by using evanescent wave detection. The devices are fabricated by standard Silicon CMOS microelectronics technology after a precise design for achieving a high sensitivity for biosensing applications. Two integrated Mach-Zehnder interferometric (MZI) devices, using two technologies, have been developed: (a) a MZI Microdevice based on ARROW waveguide (b) a MZI Nanodevice based on TIR waveguide. Direct biosensing with both sensors has been tested, after a specific receptor coupling to the surface device using nanometer scale immobilization techniques. Further integration of the microoptical sensors, the microfluidics, the photodetectors and the CMOS electronics will render in a lab-on-a-chip microsystem.